The invention relates to contact or intraocular lenses to correct the vision resulting from a possibly myopic or hyperopic and/or possibly presbyopic astigmatic eye.
It is known that myopia or hyperopia is generally corrected thanks to a spherical surface whose center of curvature must be on the optical axis of the lens, where the parameter that is usually used to define the correction to be made is the introduced spherical optical power, generally called xe2x80x9csphere value.xe2x80x9d
It is also known that the correction of presbyopia is advantageously obtained thanks to a complex surface which procures a progressive simultaneous vision correction, that is a correction whose spherical optical power varies delicately (and not abruptly) between the center and the periphery of the correcting zone, so that several images are simultaneously formed on the retina, the useful image being selected as a result of sorting by the cortex.
It is also known that the correction of astigmatism is generally obtained thanks to a toric surface whose symmetrical plane must be oriented along a meridian plane of the eye to be corrected, that is along a plane which contains the optical axis of this eye, where the parameters that are usually used to define the correction to be made are, on the one hand, the angular separation between the meridian plane along which the toric surface must be oriented and the reference meridian plane, corresponding to the horizontal meridian plane when the wearer of the lens is standing, this angular difference usually referred to as xe2x80x9cthe axisxe2x80x9d of the correction, and, on the other hand, by the cylindrical optical power introduced, usually called xe2x80x9ccylinder value.xe2x80x9d
For the toric surface of a contact lens to remain correctly positioned with regard to the eye, a means for the angular stabilization of the lens with respect to the eye must be provided, and, notably, a ballast prism which makes it possible to maintain the lens in position thanks to the weight, or bosses as described in French Patent No. 2,760,853, which use the dynamic effect produced by blinking the eyelids so that the lens remains permanently correctly positioned, or once again a progressive thinning or lightening at the top and bottom of the lens can be provided, along a direction which must correspond to the vertical direction of the eye, as described in U.S. Pat. No. 4,095,878, or a stabilization means which comprises a ballast and lightening at the top part of the lens, as described in U.S. Pat. No. 4,324,461. All these stabilization means, with the exception of the ballast prism, are located outside of the correcting portion of the contact lens, being located in the center of the lens at the level of the pupil of the eye to be corrected, for example, inside a circle having a radius of 4 mm centered about the optical axis of the lens.
Each one of these means for angular stabilization yields good results, the contact lens remaining, overall, correctly oriented. However, while the different orientations successfully assumed by the lens over time have a mean value which corresponds to the desired value, the amplitude of the variations in orientation around this mean value is such that at times the angular displacement produces a significant degradation of the optical performances.
Such a problem with angular stabilization does not arise with intraocular lenses, which are made in the form of an internal implant in the eye equipped with holding hooks which project radially but can nevertheless produce an angular displacement with regard to the nominal position during placement of the implant.
In order to correct the angular displacement, U.S. Pat. No. 5,570,143 proposes to apply to the toric lenses the techniques that are usually used to prevent spherical aberrations, techniques which have the effect of improving the depth of field where the proposed toric lens presents over at least one surface an optical topography which induces an effect of depth of field having a spherical power which would be sufficiently high to correct for the optical effect of the angular positioning separation.
U.S. Pat. No. 5,652,638 proposes a similar solution, except that the traditional techniques for improving depth of field are replaced by concentric rings.
U.S. Pat. No. 5,796,462 proposes to obtain the effect of depth of field thanks to a lens in which one of the surfaces is of the toric type, in which the meridians are not circles but curves belonging to the family of the classic cone shapes used to prevent spherical aberrations. The other face of the lens is spherical or has concentric rings.
The present invention is intended to reduce the effects of the angular displacement on toric contact or intraocular lenses by provision of a correcting portion characterized by one or more novel constructions. Per convention, the expression xe2x80x9coptical pathxe2x80x9d designates the difference in optical path introduced by a lens on the rays originating from a point light source located at infinity on the optical axis.
In one embodiment, the lens may be constructed with a xe2x80x9csmooth atoricxe2x80x9d aspect where the optical path through the correcting portion of the lens corrects for both astigmatism and an axisymmetric aberration other than astigmatism, there being no sudden surface discontinuity (thus, xe2x80x9csmoothxe2x80x9d). In another embodiment, the lens may be constructed with so-called xe2x80x9csectorsxe2x80x9d circumferentially arranged around the optical axis such that an optical path through the correcting portion of the lens varies as a function of the angular separation from the reference meridian plane, and the correcting portion is divided into at least two sectors having different astigmatism correction axes. In either embodiment, the correcting surface may be provided on either or both of the anterior or posterior faces of the lens, and the optical performance of the lens in case of angular displacement (the xe2x80x9cangular misalignment tolerancexe2x80x9d) is increased. Specifically, the angular misalignment tolerance is increased by at least 30% over a standard toric lens of the same class and in the same conditions (i.e., the same cylinder and pupil diameter).
The contact or intraocular lenses of the present invention are best described by the desired optical path through the correcting portion, and it will be understood by those of skill in the art that there are a number of ways to construct the correcting portion so as to produce such an optical path. For example, definition of the particular shape of the lens enables a mold die of that shape to be formed. Lens machining tools may also be used. Whatever the method of manufacture, the present invention is intended to encompass lenses formed to have a particular optical path.
For this purpose, the present invention provides a contact or intraocular lens comprising a correcting portion to correct the vision of a possibly myopic or hyperopic and/or possibly presbyopic astigmatic eye, comprising an optical axis and a reference meridian; characterized in that it introduces, for correction of only the astigmatism, an optical path varying as a function of the distance with regard to the optical axis and as a function of the angular separation with regard to the reference meridian, at least when this distance is between 0.4 mm and 2.4 mm, according to the following equation:
xcex4A(h,xcex8)=xcex4toric(h,xcex8)+xcex4atoric(h,xcex8)
in which equation:
xcex4toric(h,xcex8) is a cylindrical optical path which satisfies, according to the parabolic approximation, the expression xcex4toric(h,xcex8)=C/2 h2 sin2(xcex8xe2x88x92xcfx86), where xcfx86 is the axis required to correct the astigmatism of said eye expressed as an angular separation with regard to said reference meridian, and where C is the cylinder value required to correct the astigmatism of said eye; and
xcex4atoric(h,xcex8) is an optical path, such that, when h is a constant, it varies as a function of xcex8 with a 2xcfx80 period, and differently from sin2(xcex8xe2x88x92xcfx86) where this optical path in addition satisfies the condition:
xcex94xcfx86xe2x80x2xe2x89xa71.3xcex94xcfx86
which expresses the 30% increased xe2x80x9cangular misalignment tolerance,xe2x80x9d and in which in equation:
xcex94xcfx86 is the amplitude of the range of variation [xe2x88x92xc2xdxcex94xcfx86, xc2xdxcex94xcfx86] of a variable x, which amplitude is such that, for any value of x in this interval, the following condition proves correct:
MTFa[xcex4toric(h,xcex8xe2x88x92x)xe2x88x92xcex4toric(h,xcex8)]xe2x89xa7MTFa[0.25h2/2]
xcex94xcfx86xe2x80x2 is the amplitude of the range of variation [xe2x88x92xc2xdxcex94xcfx86xe2x80x2, xc2xdxcex94xcfx86xe2x80x2] of a variable x, which amplitude is such that, for any value in this interval, the following condition proves correct:
MTFa[xcex4A(h,xcex8xe2x88x92x)xe2x88x92xcex4toric(h,xcex8)]xe2x89xa7MTFa[0.25h2/2]
The notation MTFa[f(h,xcex8)] designating, for an optical path f(h,xcex8), the calculated optical quality criteria arising from the modulation transfer function produced by this optical path, for a predetermined pupil diameter, between 4 and 7 mm, according to the formula:             MTFa      ⁡              [                  f          ⁡                      (                          h              ,              θ                        )                          ]              =                  ∫                  v          =          5                          v          =          15                    ⁢                        ∫                      χ            =            0                                χ            =            360                          ⁢                                            MTF              360                        ⁢                          xe2x80x83                        [                          f              ⁡                              (                                  h                  ,                  θ                                )                                      ]                    ⁢                      (                          v              ,              χ                        )                    ⁢                      ∂            v                    ⁢                      ∂            χ                                ⁢      xe2x80x83  
in which formula xcexd and "khgr" are the polar coordinates in the plane of the angular frequencies, xcexd being expressed in cycles per degree and "khgr" in degrees; and in which MTF[f(h,xcex8)](xcexd,"khgr") is the modulation transfer function of the optical path f(h,xcex8) according to said polar coordinates.
Again, the expression xe2x80x9coptical pathxe2x80x9d used above, and more generally in the present specification, more specifically designates the difference in optical path introduced by a lens on the rays originating from a point light source located at infinity on the optical axis, so that the phase shift "psgr" introduced by this lens is connected to the optical path xcex4, in the sense of the present specification, by the relationship:
"psgr"=2xcfx80xcex4/xcex
where xcex is the wavelength of the light rays, the negative values of xcex4 and "psgr" correspond to a delay introduced on the optical wave, and the positive values to an advance.
Naturally, this optical path is valid for wavelengths located in the visible light range, and notably for the reference wavelength 550xc3x9710xe2x88x929 m.
In practice, "psgr"(h,xcex8), and more generally the phase shift or the optical path introduced by the lens can be determined by interferometry or by another method for measuring the optical phase shift, or they can be derived from the analysis of the wave front originating from the lens, realized by means of a Shack-Hartmann analyzer or another type of wave front analyzer.
The modulation transfer function, MTF, can be calculated from the phase shift or the optical path for a given pupil size according to well known algorithms (reference: M. Born, E. Wolf, Principles of Optics, Sixth Edition, Ed. Pergamon Press, p. 480 (1980)). MTFa, calculated by integration of MTF according to the formula indicated above, is a numerical criterion which allows the characterization of the performances of an optical system in good correlation with the subjective quality of the images produced by this system (reference: P. Z. Mouroulis, X. Cheng, Optical Engineering) 33:2626-2631, 1994).
It should be noted that the term 0.25 h2/2 corresponds to a defocusing of 0.25 diopter (D). The MTFa threshold appearing above is chosen as the value obtained when one considers a pure spherical defect of 0.25 D. It is assumed that a defocusing of 0.25 D with a 6 mm lens is just detectable by the wearer (D. A. Atchison et al., Subjective depth-of-focus of the eye, Optom. Vis. Sci. 74:511-520, 1997). The corresponding value of MTFa is thus chosen as minimum acceptable value. According to the invention, this threshold allows the definition of the acceptable angular tolerance of a lens intended for correcting astigmatism.
It should be noted that in each one of the above-mentioned U.S. Pat. Nos. 5,570,143, 5,652,638 and 5,796,462, the modification proposed with regard to classic toric lens concerns the radial dependence of the shape of one of the surfaces, that is the dependence according to the variable h, while the present invention proposes to work on the angular dependence (variable xcex8), alone, or in combination with the radial dependence.
More specifically, the modification with regard to a classic toric lens, which modification is represented by xcex4atoric, such that, when h remains constant and xcex8 varies, xcex4atoric(h,xcex8) does not remain constant, but fluctuates with a periodicity in xcex8 of 360xc2x0 (2xcfx80), differently from sin2(xcex8xe2x88x92xcfx86)
Thus, in the lens according to the invention, the nonaxisymmetric component is not exclusively toric.
In addition, it should be noted that each of the U.S. Pat. Nos. 5,570,143, 5,652,638 and 5,796,462, quantifies the modification made with regard to a conventional toric lens solely on the basis of optical powers.
In contrast, according to the present invention, the modification effected with regard to a classic toric lens is not evaluated on the basis of an optical power criterion, but on the basis of the optic quality criterion represented by MTFa.
It should be noted that until now MTFa was used for optical instruments, and not for the system formed by the eye and by a lens.
According to a first preferred embodiment, the term xcex4A(h,xcex8) satisfies the equation:             δ      A        ⁡          (              h        ,        θ            )        =            ∑              i        ∈        N              ⁢                            β          i                ⁡                  (          h          )                    ⁢              xe2x80x83            ⁢              cos        ⁢                  xe2x80x83                [                  i          ⁡                      (                          θ              -              φ                        )                          ]            
in which equation:
N is the set of whole numbers; and
xcex2i(h) is a set of functions satisfying the following condition:             ∑              i        ∈                  N          xe2x80x2                      ⁢          [                        (                                    1                                                h                  max                                -                                  h                  min                                                      ⁢                                          ∫                                  h                  min                                                  h                  max                                            ⁢                                                                    2                    ⁢                                                                  β                        i                                            ⁡                                              (                        h                        )                                                                                                  h                    2                                                  ⁢                                  ⅆ                  h                                                              )                2            ]        ≥      0.005    ⁢          xe2x80x83        ⁢          m              -        2            
in which equation Nxe2x80x2 is equal to N excluding 0 and 2, while hmin and hmax are the minimum distance and the maximum distance, respectively, with regard to the optical axis of the zone of the correcting portion provided to correct the astigmatism.
It should be noted that xcex4A(h,xcex8) is thus expressed according to its Fourier series decomposition in cosine. In practice, this series is convergent, so that N can be likened to whole numbers from zero to several tens.
In the case where xcex2i(h) is of the type             h      2        2    ⁢      α    i  
it should be noted that if xcex1i is a constant, the term       1                  h        max            -              h        min              ⁢            ∫              h        min                    h        max              ⁢                            2          ⁢                                    β              i                        ⁡                          (              h              )                                                h          2                    ⁢              ⅆ        h            
corresponds to xcex1i, whereas if xcex1i is not a constant the above term corresponds to the weighted mean value of xcex1i over the range of variation of h.
Given that the components of the function xcex4A(h,xcex8) for the indexes i=0 and i=2 jointly represent a correction of the spherocylindrical type, the fact that the sum of the squares of the mean coefficients xcex1i other than for i=0 and i=2 is different from 0, is characteristic of the fact that the correction provided by the lens according to the a invention comprises a nonaxisymmetric component obtained by a correction other than toric, that is atoric.
The value of 0.005 mxe2x88x922 for this sum corresponds to a minimal threshold, determined experimentally, and it is preferred to use values above this to obtain a significant optical effect.
In a structural shape preferred because of its simplicity, each of the functions xcex2i(h) satisfies the equation:             β      i        ⁡          (      h      )        =                    h        2            2        ⁢          α      i      
in which equation xcex1i, for ixcex5N, is a constant coefficient.
It should be noted that, if the correction had been purely spherocylindrical, then             α      0        =                  P        VL            +              C        2                        α      2        =          -              C        2            
where PVL is the sphere power.
In a first preferred example of this structural shape, the optical path xcex4A(h,xcex8) satisfies the equation:             δ      A        ⁡          (              h        ,        θ            )        =                    C        +        c            2        ⁢          h      2        ⁢                  sin        2            ⁡              (                  θ          -          φ          +          η                )            
in which equation xcex7 is equal to "psgr" for xcex8xe2x88x92xcfx86 between 0xc2x0 and 180xc2x0, and equal to xe2x88x92"psgr" for xcex8xe2x88x92xcfx86 between 180xc2x0 and 360xc2x0, c and "psgr" being predetermined constants. This divides the lens into 180xc2x0 xe2x80x9csectorsxe2x80x9d having different astigmatism correction axes.
More specifically, the correcting portion of the lens according to the invention is divided into two sectors separated by the reference meridian plane, the correction axis of one of the sectors being inclined with regard to a conventional toric lens, by the angle "psgr", in a first direction, whereas the correction axis of the other sector is inclined from the angle xe2x88x92"psgr" in the other direction.
Thus, for example, if one has an angular displacement of the lens of 5xc2x0 with regard to its ideal position, and the angle "psgr" is 8xc2x0, one of the sectors is separated by 3xc2x0 from the ideal and the other sector by 13xc2x0.
It is possible, as will be seen below, to achieve the result that the overall image obtained on the retina by these two sectors is of better quality, conforming to the criterion MTFa, than that which one would have with the same angular displacement for a purely toric lens.
It should be noted that the best results are not obtained using the exact cylinder power C, but with a slightly different power equal to C+c.
It is preferred that, taking into account the good results obtained, the constants c and "psgr" have, depending on the value of C, the values given by the table below, at xc2x10.125 diopter (D) for C+c, and at xc2x11xc2x0 for "psgr":
These values are particularly suitable for a pupil diameter of 6 mm.
Preferably, for the same reasons, the constants c and "psgr" have, depending on the value of C, the values given in the table below, at xc2x10.125 diopter (D) for C+c, and at
These values are particularly suitable for a pupil diameter of 8 mm.
It is also preferred for the same reasons and conforming to an experimentally evident law, that constant c is equal to zero and constant "psgr" takes on the value given by the formula below, to xc2x11xc2x0:   ψ  =      114          C      ·      DP      
in which formula DP is the pupil diameter, expressed in millimeter (mm), "psgr" being expressed in degrees (xc2x0) and C in diopters (D).
In an alternate preferred second example of this structural shape, the optical path xcex4A(h,xcex8) satisfies the equation:             δ      A        ⁡          (              h        ,        θ            )        =                    C        +        c            2        ⁢          h      2        ⁢                  sin        2            ⁡              (                  θ          -          φ          +          η                )            
in which equation xcex7 is equal to "psgr" for xcex8xe2x88x92xcfx86 between 0xc2x0 and 90xc2x0, and for xcex8xe2x88x92xcfx86 between 180xc2x0 and 270xc2x0, and equal to xe2x88x92"psgr" for xcex8xe2x88x92xcfx86 between 90xc2x0 and 180xc2x0, and for xcex8xe2x88x92xcfx86 between 270xc2x0 and 360xc2x0, c and "psgr" being predetermined constants.
Thus, the correcting portion of the lens according to the invention is divided into four sectors, separated by the reference meridian plane and by the meridian plane orthogonal to the latter plane, the correction axis of the sectors being alternately inclined, with regard to a conventional toric lens, by the angle "psgr" in a first direction and xe2x88x92"psgr" in the other direction.
Thus, it is possible to obtain, overall, on the retina, with these four sectors, an image of better quality, conforming to the criterion MTFa, than that which one would obtain for the same angular displacement with a purely toric lens.
As for the first preferred example disclosed above, the best results are not obtained with, very precisely, the cylinder power C, but with a slightly different power equal to C+c.
It should also be noted that the contact lens according to the present preferred example offers, with regard to the lens according to the preferred example with two opposite sectors disclosed above, the advantage of being less sensitive to decentering.
The centering errors in fact mutually compensate each other by the opposite sectors.
Preferably, taking into account the good results obtained, the constants c and "psgr" have, depending on the value of C, the values given in the table below, at xc2x10.125 diopter (D) for C+c at xc2x11xc2x0 C. for "psgr":
These values are particularly suitable for a pupil diameter of 6 mm. same reasons, the constants c and "psgr" have, depending on the n in the table below, at xc2x10.125 diopter (D) for C+c, and at xc2x11xc2x0 C. for "psgr":
These values are particularly suitable for a pupil diameter of 8 mm.
It is also preferred, for the same reasons, and according to an experimentally observed law, that the constant c is equal to zero and the constant "psgr" takes on the value given by the formula below to xc2x11xc2x0:   ψ  =      90          C      ·      DP      
in which formula DP is the pupil diameter, expressed in millimeter (mm), "psgr" being expressed in degrees (xc2x0) and C in diopters (D).
In a second preferred embodiment, the term xcex4A(h,xcex8) satisfies the equation:             δ      A        ⁡          (              h        ,        θ            )        =            ∑              i        ∈        E              ⁢                            β          i                ⁡                  (          h          )                    ⁢              xe2x80x83            ⁢              cos        ⁢                  xe2x80x83                [                  i          ⁡                      (                          θ              -              φ                        )                          ]            
in which equation:
E is a finite set comprising the whole numbers starting from 0; and
xcex2i(h) is a set of functions satisfying the following condition:             ∑              i        ∈                  E          xe2x80x2                      ⁢          [                        (                                    1                                                h                  max                                -                                  h                  min                                                      ⁢                                          ∫                                  h                  min                                                  h                  max                                            ⁢                                                                    2                    ⁢                                                                  β                        i                                            ⁡                                              (                        h                        )                                                                                                  h                    2                                                  ⁢                                  ⅆ                  h                                                              )                2            ]        ≥      0.005    ⁢          xe2x80x83        ⁢          m      2      
in which equation Exe2x80x2 is equal to E disqualifying 0 and 2, while hmin and hmax are the minimum distance and the maximum distance, respectively, with regard to the optical axis of the zone [sic] of the correcting portion provided to correct the astigmatism.
In a first preferred structural shape of this second embodiment, because of its simplicity, each one of the functions xcex2i(h) satisfies the equation:             β      i        ⁡          (      h      )        =                    h        2            2        ⁢          α      i      
in which equation each xcex1i, for ixcex5E is a constant coefficient.
This preferred structural shape notably corresponds to one of the examples with two or four sectors, disclosed above, with low pass filtering for the optical path, which consists in keeping only the first coefficients xcex1i, for example, only up to i=10, or only up to i=3, and with an optimization of these coefficients.
Thus one prevents the existence, on the lens, of a crest between the sectors whose correction axis is differently inclined, which facilitates the manufacture of the lens and prevents wearer discomfort which could be produced by such a crest.
Preferably, because of the good results obtained based on a lens according to the first preferred example disclosed above, with two opposite sectors, the set E comprises the whole numbers from 0 to 10, and the coefficients xcex1i have, as a function of C, values such that they satisfy the inequation:                     ∑                  i          ∈          E                    ⁢                        (                                    α              i                        -                          α              i              xe2x80x2                                )                2              ≤      0.05    ⁢          xe2x80x83        ⁢          m              -        1            
the coefficients xcex1ixe2x80x2 having the values given in the table below:
These values are particularly suitable for a pupil diameter of 6 mm.
Preferably, for the same reasons, still based on a lens according to the first preferred example, disclosed above, with two opposite sectors, the set E comprises the whole numbers from 0 to 10, and the coefficients xcex1ixe2x80x2 have, as a function of C, values such that they satisfy the inequation:                     ∑                  i          ∈          E                    ⁢                        (                                    α              i                        -                          α              i              xe2x80x2                                )                2              ≤      0.05    ⁢          xe2x80x83        ⁢          m              -        1            
the coefficients xcex1ixe2x80x2 having the values given in the table below:
These values are particularly suitable for a pupil diameter of 8 mm.
It is also preferred, based on a lens according to the second preferred example disclosed above, with four sectors, that the set E comprises the whole numbers from 0 to 10, and the coefficients xcex1i have, as a function of C, values such that they satisfy the inequation:                     ∑                  i          ∈          E                    ⁢                        (                                    α              i                        -                          α              i              xe2x80x2                                )                2              ≤      0.05    ⁢          xe2x80x83        ⁢          m              -        1            
the coefficients xcex1ixe2x80x2 having the values given in the table below:
These values are particularly well suited for a pupil diameter of 6 mm.
Preferably, for the same reasons, still based on a lens according to the second preferred example disclosed above, with four sectors, the set E comprises the whole numbers from 0 to 10, and the coefficients xcex1i have, as a function of C, values such that they satisfy the inequation:                     ∑                  i          ∈          E                    ⁢                        (                                    α              i                        -                          α              i              xe2x80x2                                )                2              ≤      0.05    ⁢          xe2x80x83        ⁢          m              -        1            
the coefficients xcex1ixe2x80x2 having the values given in the table below:
These values are particularly well suited for a pupil diameter of 8 mm.
In a second structural shape of the second embodiment, each one of the functions xcex2i(h) satisfies the equation:             β      i        ⁡          (      h      )        =                    h        2            2        ⁢          α              i        ,        j            
in which equation j is a whole number which varies as a function of h, in stages, and each xcex1ij, regardless of what i and j are, is a predetermined constant coefficient.
Naturally, each stage corresponds to an annular zone of the correcting portion of the lens.
This structural shape is well adapted to the fact that the diameter of the pupil varies depending on the individuals and the conditions of vision (primarily lighting and proximity).
In a third structural shape of the second embodiment, each one of the functions xcex2i(h) satisfies the equation:             β      i        ⁡          (      h      )        =                    h        2            2        ⁢                  ∑                  j          =          0                M            ⁢                        α                      i            ,            j                          ⁢                  h          i                    
in which equation M is a predetermined whole number and each xcex1i, regardless of what i and j are, is a predetermined constant coefficient.
This structural shape is similar to the preceding one except that, instead of having functions xcex2i(h) which vary as a function of h in stages, they vary progressively and gently.
The invention also relates, according to a second aspect, to a method for the preparation of a contact or intraocular lens as disclosed above, characterized in that it comprises:
a) a step of determination of the optical path that must be introduced by the correcting portion of this lens;
b) a step of selection of the shape of the posterior surface of said correcting portion, from a series of predetermined shapes, in order to procure optimal comfort for the wearer of the lens;
c) a step of determination of the shape of the anterior surface of said correcting portion, from said shape selected for the posterior surface in step b) and from the optical path determined in step a); and
d) a step of manufacturing said lens with the correcting portion presenting the anterior surface and the posterior surface which were so determined.
Such a method for the preparation of the lens is particularly well suited for fabrication by direct machining: the posterior surface of the lens can have a relatively simple shape, and all the complexity can be transferred to the anterior face, in particular so that the correcting portion introduces the required optical path, and, in the case of a contact lens, to equip this lens with angular stabilization means.
The invention also relates, still according to its second aspect, to an alternate method for the preparation of a contact or intraocular lens as presented above, characterized in that it comprises:
a) a step of determination of the optical path that must be introduced by the correcting portion of this lens;
b) a step of selection of the shape of the anterior surface of said correcting portion from a series of predetermined shapes, each being axisymmetric;
c) a step of determination of the shape of the posterior surface of said correcting portion from said shape selected for the anterior surface in step b) and from the optical path determined in step a);
d) a step of manufacturing said lens with said correcting portion presenting said posterior surface and anterior surface which were so determined.
This preparation method is particularly well suited for making lenses by molding.
Indeed, because the angular maintenance means, that is, in the case of a contact lens, the angular stabilization means, are generally located on the anterior surface of the lens, one has, in this embodiment, a situation where the angular maintenance means and the surface procuring the correction of astigmatism are located on the anterior side and the posterior side, respectively.
This allows, for the same cylinder value and the same sphere value, realization of all the axes using the same two half molds, where the different values of astigmatism axes are obtained by turning one half mold with regard to the other.